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This invention relates generally to the field of frequency planning in wireless communication networks and, in particular, to a system and method for improved consideration of the impact of various factors in frequency planning.
In wireless communication systems, such as a cellular mobile radio communication system, the geographic area served by the system is divided into geographically defined cells. In the system, there is a finite number of carrier frequency channels, typically radio-frequency (RF) channels, that are available for use during communications in and between network areas cells. Typically, a frequency group, consisting of a subset of all of the available frequencies, is assigned to each cell for that cell""s use during communications.
However, because the number of frequency groups is limited, it is necessary to reuse them within the area served by the system. To provide for a greater coverage by the system, and to provide for greater capacity through higher reuse of frequency groups, the network service area cells may be further divided up into sectors. Because the use of a particular frequency by two different sectors can result in interference during a call or even result in the call being completely cut-off or xe2x80x9cdroppedxe2x80x9d, an effort must be made to assign frequency groups to sectors in a manner that minimizes the amount of interference.
In wireless communication networks, there are two primary types of interference: co-channel interference and neighbor or adjacent channel interference. Co-channel interference is the interference from communication sources tuned to the same frequency as the operating channel. Adjacent-channel interference comes from communication sources using channels near the operating channel in the frequency domain. To achieve the desired voice or data transmission quality, the ratio of the received signal over the combined co-channel and neighbor-channel interference must be above a specified threshold. Such channel interference can be up-link or down-link interference or a combination of these interferences. Down-link interference is channel interference received at regions serviced by a first base station caused from signals transmitted by other base stations. Up-link interference is interference at the base stations caused from signals transmitted by mobile units in regions of the coverage area that are not serviced by that base station.
Furthermore, other factors such as antenna patterns, power levels, scattering, and wave diffraction variations combined with buildings, various other structures, hills, mountains, foliage, and other physical objects contribute to the interference experienced during wireless communications.
In frequency planning for a wireless communication network, the primary task is to try to predict and attempt to reduce the amount of channel interference experienced by strategically assigning certain channels to certain sectors. Typically, this can be achieved by assigning frequencies so that the distance between co-channel and adjacent channel sectors is maximized. In this context, xe2x80x9cdistancexe2x80x9d does not necessarily refer to geographic distance but connotes a distance in the RF sense. That is, although sectors far apart from each other geographically are less likely to xe2x80x9cseexe2x80x9d each other, they can still interfere with each other. For example, a high sector can interfere with a sector as far as hundreds of miles away. Maximizing this distance decreases the chances of the sectors conflicting with one another in the airwaves. However, a severe consequence of maximizing this distance is that it effectively reduces the amount of channel combinations possible in the network service area, thereby limiting the amount of coverage available for wireless communication. Typically, frequency planning is ordinarily accomplished by three primary techniques including channel sets, reuse patterns and pixel based interference analysis.
Channel sets are non-overlapping subsets of the available channels organized according to a periodic frequency spacing in terms of number of channels between, members of a given set. The principal disadvantage of using channel sets is that the number of channels required from sector to sector usually varies, and optimal frequency planning will require that just that number, rather than the number in an arbitrary set, be assigned to each sector.
In the reuse pattern scheme, the sectors in a network are arranged in a two-dimensional pattern, or xe2x80x9cgridxe2x80x9d. Channels or, more commonly, channel sets, are then assigned so that co-channel or adjacent channel assignments appear periodically in different sectors. The primary disadvantage of frequency planning based upon a reuse grid is that, within a given network, varying terrain and man-made xe2x80x9cclutterxe2x80x9d, such as buildings and other structures, will affect the characteristics of radio propagation and attenuation. Therefore, adhering to a fixed and rigid co-channel or adjacent channel spacing on a grid will likely provide inadequate isolation in some cases, resulting in excessive interference, and more than the required isolation in others, thereby reducing reuse efficiency. Furthermore, in addition to less than optimal interference levels, the fixed reuse approach results in much reduced capacity in many parts of the network where frequencies can be added freely due to an RF shield, such as a mountain ridge, but the grid prohibits such an assignment.
In pixel based interference analysis, the entire network service area is divided into a large number of very small xe2x80x9cpixelsxe2x80x9d or xe2x80x9cbinsxe2x80x9d. In one example, each pixel would be a 100 meter square, so that a network service area of 100 kilometers by 100 kilometers would contain 1 million pixels. For each pixel, a system engineer will ascertain the strongest incident signal level from the sectors nearby and then the incident signal levels from each of the other sectors in the network to determine potential interferences. From this information, the system engineer can determine the predicted levels of co-channel or adjacent channel interference that would be present in that pixel if certain sectors were assigned, respectively, the same radio channels as the serving sector or channels adjacent to those in the serving sector.
However, pixel-by-pixel interference analysis also has many significant limitations. While pixel by pixel analysis can predict interference problems that are likely to result from a proposed frequency plan, it does not provide any such plan in the first place, nor does it inherently suggest modifications to a frequency plan that would reduce interference.
Furthermore, there is an inherent limitation on the amount of data that can be presented in pixel by pixel interference analysis. At the same time, pixel by pixel analysis produces an amount of data which is not easily susceptible to human interpretation. Finally, because conventional pixel by pixel interference analysis relies solely on predicted levels, it carries over the inaccuracies in such data as described above and results in erroneous frequency assignments.
Thus, while these existing techniques can provide for some measure of protection and relief from channel interference in the network service area, they still fail to account for the many variables and factors which can affect wireless communications on a day to day basis. Accordingly, it would be desirable to have a system and method for frequency planning within a wireless communication network which accounts for the many variables and factors affecting the quality of wireless communications, reduces the interference experienced during wireless communication, and does not limit the coverage area of network cells.
The present invention provides a system and method for creating an impact matrix for use in allocating frequency channels in a wireless communication network service area which is divided into a plurality of sectors and further divided into a plurality of pixels. The impact matrix provides impact scores which characterize the impact of making certain co-channel or adjacent channel assignment in pairs of sector by sector within a network service area.
The impact scores are developed by a series of steps, the first of which involves selectively merging signal propagation analysis data and empirical measurement data to determine an anticipated signal level for each one of the plurality of pixels in the network area. Once the signal levels within each pixel are obtained, a determination is made as to which sector within the network service area is serving that particular pixel. The system then assigns a weighted channel interference impact score for the pixel based on the interference between the pixel""s signal serving sector and signals from each of the other non-serving sectors in the network area. Overall sector by sector impact scores based on the weighted channel interference impact scores are determined for all of the pixels for which a sector is the serving sector.
In one embodiment of the present invention, the signal level data from the signal propagation analysis, empirical measurement, and switch logs analysis is merged according to user assigned confidences for the data. The step of determining a weighted channel interference impact score between the pixel""s signal serving sector and signals from each of the other sectors in the network area includes calculating a ratio between a signal level from the serving sector and signal levels from each of the other sectors in the network area, assigning interference impact scores for each of the other sectors in the network area, and adjusting the interference impact scores according to user assigned factors such as network area sensitivity to call quality and amount of call volume for that network area.
Once the impact matrixis developed, the scores-in the matrix may be modified to further accurately characterize the signal impacts of interferences within a network service area. The impact matrix scores may be adjusted according to data which may have been previously input to the system or may be contemporaneously input. Such data includes channel pairing relationships among sectors which are known to provide low levels of excessive interference, channel pairing relationships among sectors which are known to provide high levels of excessive interference, and detailed call history information. Detailed call history information can include data on dropped calls and associated sector and channel combination where call drops occur.
The impact matrix may then be used for frequency planning in the network service area. The impact matrix will provide a user, typically a network engineer, with a way to predict the signal quality impact of certain channel assignments within the network service area.